Which benthivorous fishes feed greedily after bottom trawling?

Type:

Master Thesis subject (30 ECTS)

programme:

EMBC+

Chronic bottom trawling has resulted in a long-term change of species and size composition of benthic communities towards highly productive small and short-lived species at the expense of low-productive larger and long-lived organisms. The alteration of benthic communities induces a change in the food availability for demersal fish communities, promoting fish that feed upon smaller opportunistic species, like sole, plaice and dab. Besides the long-term effect, the passage of a beam trawl also causes direct, short-term disruptions. Some benthic invertebrates, such as molluscs, are not easily available for demersal fish, but may become more accessible after a trawl has passed. Bottom trawling exposes them to physical disturbances, increasing their sensitivity to predation and scavenging because of physical injuries and unravelling them from their refuges. Hungry benthivorous fish can take short-term advantages and feed upon exposed benthic organisms. The effect is immediate, and depends on the competitive capacities of demersal fish.
Beam trawl fishery is currently changing its seafloor impact by replacing the tickler chains with electric pulses to catch fish or shrimps. These effects are not quantified hitherto, nor are the changes of the short-term exposure of benthic fauna to feeding of benthivorous fish. In this Master’s thesis, we will quantify the changes in feeding behaviour of benthivorous fish after trawling a benthic community with a conventional and a pulse shrimp beam trawl. Several benthivorous flatfish species will be sampled in fishing grounds in the southern North Sea, enabling analysing their competitive capacities for scavenging. The species composition of their diet will be determined, as well as the stomach fullness. Samples will be taken from the first haul of the beam trawl, but also after trawling the area. A comparison of the diet before and after trawling will be evaluated for the two types of fishing gear, the conventional and alternative pulse shrimp trawl. A correlation between the benthic community, the fish diet and trawling intensities will be sought for, indicating the potential implication of trawling on feeding. Fish diets depend on the body sizes, which will be considered as well throughout the analysis.